1. Field of the Invention
The present invention relates to a device for terminating or splicing a cable used for towing air/water guns behind a seismic vessel, called a gun cable, which is described in Norwegian Patent Application No. 83 1546, corresponding to U.S. patent application Ser. No. 601,598, filed Apr. 18, 1984. The termination has the following main functions:
(a) It should be able to transmit peak load tensions of up to 10 tons from the cable to the towed gun array or to the next cable section in such a manner that only the reinforcing wire is subjected to loading;
(b) It should provide a watertight connection for transfer of the signals from the conductors in the cable to the associated conductors in the next cable section or in the guns via underwater connectors;
(c) It should be able to connect compressed air from the air hose in the cable to the guns, in such manner that any air which leaks out of the hose or coupling is released directly into the surrounding water without disturbing the other functions of the termination;
(d) It should also contain room for electronics in a watertight compartment.
2. Description of the Prior Art
Several methods are used today for terminating gun cables. They all comprise terminating the cable in one or two heads (called "bulkheads"). In at least one of these methods, a casting technique is utilized to obtain a watertight seal. It has proved to be very difficult to make good gun cable terminations, which is reflected among other things in the price of these assemblies. It is not uncommon for the termination alone to cost as much as a cable over 100 meters long. Experience has also shown that when faults occur, for example if the electrical signals become crossed in a gun cable having termination, the fault occurs at least as often in the termination as in the cable itself. There are several reasons why it is so difficult to make good gun cable terminations. The most important seems to be that during use, the termination is subjected to continuously variable mechanical loads, both from the tension in the reinforcing wire and from the pulsating pressure in the air hose. These mechanical stresses, in time, cause the casting in the bulkhead to "slip" or slide away from the materials to which it originally adhered. Thus openings arise which may gradually fill with water.
Another problem with present-day terminations is the choice of different casting materials. No single casting material exists which adheres to all of the various materials used in a termination and which at the same time can provide adequate watertightness over a lengthy period of time. Polyurethane is ofter used, but it does not provide the best seal against water penetration. Separate castings with other casting materials are often used around the individual conductors to ensure maximum adherence and sealing to the conductor insulation, but the construction then becomes more complex with even more interfaces between different materials.